The invention relates to a monochrome cathode ray tube provided at one end with an electron gun and at the end located oppositely thereto with a display screen having a phosphor layer. A deflection unit is provided around the neck of the display tube and a magnetic focusing device is provided around the neck of the display tube between the electron gun and the deflection unit, which device has means for generating a static magnetic field.
When focusing electrons in a cathode ray tube, two types of lenses are used, viz. electrostatic or magnetic lenses. To achieve a good definition it is desirable to achieve a very good extent of focusing (small electron spot or high resolution). Magnetic lenses can generally be provided outside the neck of the tube, in contrast to electrostatic lenses which are present in the neck of the tube. Therefore the diameter of the magnetic lenses may be larger, which yields a better lens quality: the spherical aberration decreases with an increasing lens diameter. A smaller spherical aberration leads to a smaller spot on the screen, which is required for high-resolution guns. Consequently, a magnetic focusing lens is preferably used for high-resolution (projection) television tubes.
When magnetic focusing lenses are used, a distinction can be made between two types, viz. electromagnetic and magnetostatic lenses. In an electromagnetic lens a field is generated by a coil which is partially enclosed by a yoke. In a permanent magnetic lens the field is generated by a permanent magnetic material with or without a yoke (see German Patent 891,119). The electron beam is moved across the screen by a deflection coil, while the intensity of the beam is modulated for obtaining a picture. The large beam aperture angle which occurs at a large intensity of the electron beam results in the electron spot on the screen being not only enlarged but also distorted during deflection of the beam by the deflection coil. As a result of this deflection defocusing an elliptical spot having a diameter which is larger than that in the center of the display screen is produced on the edge of the screen.
In some uses of cathode ray tubes, such as projection television tubes or so-called data-graphics display tubes such a distortion cannot be tolerated.